Dock levelers are utilized in the transfer of goods from a building to and from a trailer by bridging the gap from the building floor to the trailer bed. Dock levelers include a frame or support structure for mounting the leveler in a pit of a loading dock. The rear end of a conventional dock leveler is hinged to the building floor. The opposite end has an extendable lip plate that pivots out and onto the trailer bed. Levelers are adapted to move from a generally horizontal position where the upper surface of the ramp is flush with the surface of the building floor to a second generally inclined position to provide a ramp between the bed of the truck and the dock floor.
In a conventional hydraulically operated dock leveler a cylinder interconnects the frame and the ramp, and acts to pivot the ramp from a horizontal dock level position to the upwardly inclined position. In addition, a second cylinder interconnects the ramp and the lip and pivots the lip from the downward pendant position to the outwardly extended position when the ramp is fully raised. The lip cylinder can alternatively be substituted with a mechanical linkage interconnecting the frame to an actuator on the ramp to extend the lip as the ramp is near its upper most inclined position. An electric motor that when actuated drives a pump providing the necessary hydraulic flow and pressure. The operator typically pushes and holds a control button until the ramp fully raises and the lip extends. Releasing the button allows the ramp to lower by gravity until the lip contacts the bed of the truck where hydraulic pressure is released and the truck holds the weight of the leveler. After loading the operator pushes the control button raising the ramp until the lip retracts pendent and releasing the button allows the dock leveler to store level with the building floor.
U.S. Pat. Nos. 4,365,374 and 4,641,388 pertain to hydraulically operated dock levelers and U.S. Pat. No. 5,088,143 pertains to a mechanical lip actuation. Other other hydraulically operated dock levelers are shown and described in U.S. Pat. Nos. 4,110,860; 4,920,598 and 4,955,923.
The two conventional hydraulic dock leveler designs each have advantages and disadvantages. The dock leveler with a separate lip extension cylinder can be efficient but is more costly to purchase and service mostly due to expensive and micro machined parts, valves, and additional hydraulic hose and fittings. It is also dependent on the correct functioning of a sequencing valve for the lip to extend and retract. This sequencing valve can come out of adjustment or get contaminated causing it to malfunction and not extend or retract the lip. A further disadvantage of a hydraulic lip cylinder is that it causes resistance to the retraction of the lip especially as the dock leveler ages and the lip hinge rusts. The mechanical lip counterpart is less costly but requires a latching and de-latching mechanism that has many parts that wear requiring regular maintenance and repair. If the mechanical latch does not release the dock cannot be stored and the mechanism can be destroyed from impact of an incoming trailer.